1. Field of the Invention
The present invention relates to integrated optical metrology, and more particularly to improving the integrated optical metrology serviceability and availability by optimizing the design of Field Replaceable Units (FRU) for the integrated optical metrology system.
2. Description of the Related Art
In the manufacturing of integrated circuits, very thin lines or holes down to 45 nm or sometimes smaller are patterned into photoresist and then transferred using an etching process into a layer of material below on a silicon wafer. It is extremely important to inspect and control the width and profile (also known as critical dimensions or CDs) of these lines or holes. Traditionally the inspection of CDs that are smaller than the wavelength of visible light has been done using expensive and slow scanning electron microscopes (CD-SEM) since all measurements are done in vacuum. As the structures get smaller and smaller, the process tolerance is getting tighter and tighter. Hence the required measurement precision and accuracy also becomes tighter and tighter. The measurement frequency and throughput also need to increase in order to monitor the process condition in real time. CD-SEM cannot meet the many CD metrology requirements in those areas due to its low throughput and limited CD profiling capability. In many cases, manufacturers need to measure CD and profiles immediately after the photoresist has been patterned, a non-destructive metrology is needed to avoid photoresist damage induced by e-beam in CD-SEM. For real time process control or advanced process control (APC), the measurement module needs to be integrated with process equipment, such as wafer track that develops the photoresist or etcher.
One measurement technique that has promise for non-destructive and fast CD measurements is scatterometry. Exemplary scatterometry techniques are described in U.S. Pat. No. 6,538,731, entitled “System and Method for Characterizing Macro-Grating Test Patterns in Advanced Lithography and Etch Processes”, by Niu, et al., issued on Mar. 25, 2003, and is incorporated in its entirety herein by reference. Exemplary scatterometry techniques are described in U.S. Pat. No. 6,433,878, entitled “Method and Apparatus for the Determination of Mask Rules Using Scatterometry”, by Niu, et al., issued on Apr. 13, 2002, and is incorporated in its entirety herein by reference. This technique takes advantage of the fact that small periodic lines or holes diffract an incident light beam, and the properties of the light in each of the diffraction orders carries information of the lines and holes. In practice, the optical properties of zero-th diffraction order that is reflected (or, for transparent samples, transmitted) from the periodic structures are measured with an optical metrology sensor, and measured data is analyzed with an analysis software, such as ODP. In performing scatterometry measurement, the intensities of the reflected or transmitted beam at various polarization states are measured versus wavelength, and in some cases, versus angle of incidence of the beam.
Optical metrology sensor measures the optical properties of the features on a wafer. These optical properties include the intensity and polarization state of reflected beam. These techniques are described in U.S. Pat. No. 7,064,829, entitled “Generic Interface for an Optical Metrology System”, by Li, et al., issued on Jun. 20, 2006, and are incorporated in its entirety herein by reference. The optical metrology sensor can be designed to sense one or more of this optical properties. For example, the tool that measures the intensity of reflected beam is called a reflectometer, and tools that measure the polarization change are called ellipsometers. The optical metrology sensor typically uses photometric or spectral photometric detectors.
An optical metrology sensor involves directing an incident beam in one or more polarization states at a feature on a wafer, measuring the resulting diffraction signals, and measuring the signal from standard reflector in reflectometer case, the measured signs are first analyzed to find the optical properties of the feature, namely reflectivity or polarization state changes. The measured optical properties of the feature are analyzed to determine various characteristics of the feature. In semiconductor manufacturing, optical metrology is typically used for quality assurance, process control, and equipment control. For example, after fabricating a periodic grating in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated. Further more, the measured dimensions of features can be used to control the process equipment work conditions.
An integrated CD measurement tool must be both fast and compact, and must be non-destructive to the wafer under test. The wafer may also be loaded into the measurement tool at an arbitrary angle creating further complications for instruments that have a preferred measurement orientation with respect to certain wafer features.
Integrated metrology tools are needed for real time process control. The reliability and availability is paramount in this scheme. Any problem in metrology module will hinder process control and may cause process tool to stop. The maintenance time of the integrated metrology modules also need to be significantly reduced to minimize the downtime of the process tool and hence to maximize the availability of the process tool.